1. INTRODUCTION

Climate changes predicted as a result of anthropogenic increases in greenhouse
gases include various specific effects of relevance to farming systems
of the Asia-Pacific region: sea level rise, higher tropical surface temperatures,
increased tropical cyclone frequency and severity, and changes in cloud
cover and precipitation. Increased ultraviolet radiation due to ozone layer
depletion by chlorofluorohydrocarbons (CFCs) may also affect elements of
farming systems.

These climatic changes have numerous implications for farming systems.
Many are rather obvious: rising sea level, in the absence of defensive
measures, would gradually inundate coastal farmland, forcing shifts to
more salt-tolerant activities like shrimp farming and, ultimately, coastline
retreat; more and stronger cyclones would cause more crop damage; and changes
in precipitation would reduce economic returns on existing water resources
infrastructural investment in affected areas, to name but three examples.
Other effects more subtle effects are also possible, such as changes in
natural plant propagation rates and species mix and in plant disease patterns.

For the future, it will be important to work towards an integrated understanding
of climatic, demographic, economic, and technological change. Methodologies
need to be developed for weaving climate change considerations appropriately
into long-term farming system monitoring programs, and into our understanding
of the future evolution of rural economies in general and of farming systems
in particular.

2. CLIMATE CHANGE

Greenhouse gases [carbon dioxide (CO2), methane, CFCs, etc.]
are increasing in the atmosphere as a result of various human activities,
including combustion of fossil and biomass fuels, deforestation, and expansion
of some types of agricultural activities. The overall global warming effects
of these increased greenhouse gases is well understood (USEPA, 1989); less
so are the regional and higher-order (e.g. hydrologic) effects. The current
scientific consensus is that global warming effects have not yet been observed,
though controversial claims to the contrary have been made (Schneider,
xx).

The CFCs, by a completely different physical mechanism, also act to
deplete the ozone layer thereby increasing the amount of UV-B radiation
reaching the earth's surface.

Most global warming predictions are made for a "2xCO2 world",
that is, for the time-frame when greenhouse gas concentrations (converted
and summed to CO2 equivalent concentration) will have reached
twice the pre-industrial level. At current emission rates, 2xCO2
will be reached in year 2030; at reduced emission rates now proposed, 2xCO2
would be delayed to about year 2060. The predictions are based on computer
models of the atmospheric general circulation, historical climate observations,
and other information.

Current scientific estimates of climate change are as follows. For global
warming-related effects, two types of numbers are given: (i) total values
for 2xCO2 conditions and (ii) the rates corresponding to 2xCO2
changes occurring the over the period 1990-2060. The predicted changes
are:

Global average changes

Surface temperature, averaged globally and annually, is expected to have
risen by between 2.8 and 5.2 C at 2xCO2 (0.028 to 0.074 C per
decade).

Sea level, averaged globally and annually, is expected to have risen by
between 0.3 and 0.8 m at 2xCO2 (4 to 11 cm per decade). Sea
level rise thereafter is expected to accelerate, rising a total of 0.5
to 2 meters between 1990 and 2100.

Rainy season precipitation will likely hold steady or increase in the wet
tropics and could decrease or become more variable in the semi-arid tropics
(while decreasing in mid-latitude summer) but climate model precipitation
results are somewhat contradictory (Crosson, 1989).

Area of occurrence, seasonal duration, and intensity of cyclones could
change. But even the sign of these changes is uncertain: frequency of tropical
cyclones in the north Indian Ocean during the last 30 years was found to
be inversely correlated with sea-surface temperature even though the energy
source for the cyclones is the heat in the surface mixed layer (McBride,
1989).

Tropical UV-B flux after ozone depletion could exceed that experienced
anywhere on earth in recent geological history, since the highest naturally
occurring UV-B fluxes are at the equator (Caldwell
et. al., 1989).

Probability effects.

Other things being equal, as mean magnitudes increase, extreme events of
a given magnitude occur more frequently. For example, in a location where
5.0 m above mean sea level (MSL) occurs once every 10 years on average
and 5.5 m once every 30 years, a sea level rise of 0.5 m will make the
5.5 m event three times more frequent. Similar arguments apply number of
temperature etc. extrema.

Other things being equal, variances increase as means increase.

3. IMPACTS OF CLIMATE CHANGE ON FARMING SYSTEMS

Current understanding of the effect of these climatic changes on farming
systems is not very far advanced, with research interest in mid-latitude
agriculture predominating in the literature. The possible impacts of climate
change on farming systems include:

Changes in water resources demand and availability. Precipitation,
evaporation, transpiration, etc., can all change. Flood control, drainage,
and irrigation infrastructure will have to evolve with the changes, which
makes longer-term (greater than 50 year life-span) developments less attractive
unless they can be updated and modified. Existing long-lived infrastructure
may be outpaced.

Longer growing season in frost-affected areas. In the Asian context,
this affects upland farmers in Nepal, Bhutan, China, etc.

Changes in self-propagation of perennials. For farming systems incorporating
naturally propagating forest or range land, changes in the rates of propagation
and species mix will likely track changes in climatic conditions (Davis,
1989).

Greater risks for monoculture. Compared to countries (and by extension
farming systems) with diversified agricultural production, those reliant
on only one or few principle commodities are at higher risk from climate
change (Riebsame, 1989), just as they are from extreme weather events,
pests, etc.

Interactions between impacts. Findings so far tend to suggest that
responses of farming system elements to climate change forcings may often
be non-linear, that is, with changes in one climate parameter (e.g. precipitation)
modulating the response to changes in other parameters (e.g. UV-B) (Tevini
and Teramura, 1989).

5. CLIMATE CHANGE IN PERSPECTIVE

In much of the Asia-Pacific region, the context within which climatic change
will occur is characterized by sweeping transformations that are proceeding
roughly an order of magnitude faster than changes in climate. Upper-bound
(and widely questioned) estimates of sea level rise imply that on the order
of 10% of Bangladesh's land area will be inundated by 2050, while conservative
population projections indicate 100% growth over this period; at the same
time, high rates of agricultural intensification, industrialization, and
urbanization are also transforming the economic and technological basis
of Bangladesh farming systems. But as increasing landlessness is a key
problem in Bangladesh, potential sea level rise should continue to be monitored.
In other regional settings, climatic change may be one of the dominant
processes. The low islands of the Indian and Pacific Oceans, much of whose
total area is very low and coastal could be disastrously affected by sea
level rise and increasing cyclone activity.

Karl, T. R., H. Diaz, and T. Barnett, 1989: Climate variations
of the past century and the greenhouse effect (a report based on the First
Climate Trends Workshop). National Climate Program Office/National Oceanic
and Atmospheric Administration (NOAA), Rockville, MD.

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